The present invention relates to a magnetoresistive effect element and a magnetic memory device, more specifically, a magnetoresistive effect element using spin transfer torque switching (spin transfer magnetization switching) mechanism, and a magnetic memory device using the magnetoresistive effect element.
Recently, as a rewritable non-volatile memory, a magnetic random access memory (hereinafter called an MRAM) including magnetoresistive effect elements arranged in a matrix is noted. The MRAM memorizes information by using combinations of magnetization directions of two magnetic layers and reads memorized information by detecting resistance changes (i.e., current changes or voltage changes) between when magnetization directions of the magnetic layers are parallel with each other and when magnetization directions of the magnetic layers are anti-parallel with each other.
As the magnetoresistive effect element forming the MRAM, a giant magnetoresistive (hereinafter called a GMR) element and a tunneling magnetoresistive (hereinafter called a TMR) element are being proposed. Among them, the TMR element which can have large resistance changes is noted as a magnetoresistive effect element to be used in the MRAM.
The TMR element includes two ferromagnetic layers laid one on another with a tunnel insulating film formed therebetween and utilizes the phenomena that the tunnel current flowing between the magnetic layers via the tunnel insulating film changes depending on relationships of magnetization directions of the two ferromagnetic layers. That is, the TMR element has low element resistance when the magnetization directions of the two ferromagnetic layers are parallel with each other and has high element resistance when the magnetization directions are anti-parallel with each other. These two states are related to data “0” and data “1” to thereby use the TMR element as a memory device.
The MRAM using the TMR element includes two signal lines (e.g., a bit line and a write word line) normally crossing each other, which are provided respectively above and below the TMR element (see, e.g., Japanese published unexamined patent application No. Hei 11-317071 (1999) (Patent Reference 1)). In reading data from the TMR element, a change of the element resistance is read to judge whether the data memorized in the TMR element is “0” or “1”. On the other hand, generally in writing data into the TMR element, current is flowed in the signal lines, and a synthetic magnetic field of magnetic fields generated by the signal lines is applied to the TMR element, whereby the magnetization direction of one magnetic layer (free magnetic layer) is changed corresponding to the applied magnetic field (current magnetic field write method).
However, in the current magnetic field write method, the demagnetizing field of the free magnetic layer is increased as the size of the TMR element is reduced so as to satisfy larger capacities exceeding, e.g., gigabits (Gbits), and the magnetization reversing magnetic field Hc of the free magnetic layer is accordingly increased. Accordingly, when the write current is small, defective write takes place. Thus, as the integration increases, the write current is increased, and electric power consumption is increased. Kohjiro Yagami et al., “Research Trend in Spin Transfer Magnetization Switching”, Journal of the Magnetics Society of Japan, Vol. 28, No. 9, 2004, pp. 937-948 (Non-Patent Reference 1) anticipates that when the memory cell size is reduced to about 100 nm so as to satisfy large capacities exceeding gigabits, the conventional current magnetic filed write method drastically increases the write current, and actually the write will be difficult.
The MRAM with selective transistors connected to must include write word lines in addition to the bit lines and the word lines, which complicates the device structure and the fabrication process.
In such background, recently as the magnetoresistive effect element forming a large-capacity MRAM, the spin transfer torque switching (STS) element is noted (refer to, e.g., Non-Patent Reference 1). The spin transfer torque switching element is a magnetoresistive effect element including an insulation layer or a non-magnetic conductor layer between two ferromagnetic layers, as do the GMR element and TMR element.
In the spin transfer torque switching element, when current is flowed from the free magnetic layer to the pinned magnetic layer perpendicularly to the film surface, spin-polarized conduction electrons flow from the pinned magnetic layer to the free magnetic layer to make the exchange interaction with the electrons of the free magnetic layer. Resultantly, torques are generated between the electrons, and when the torques are sufficiently large, the magnetic moment of the free magnetic layer is switched from anti-parallel to parallel. On the other hand, when the current is applied oppositely, the conduction electrons flow from the free magnetic layer to the pinned magnetic layer. Parts of the conduction electrons are reflected against the interface between the nonmagnetic layer (insulation layer or nonmagnetic conduction layer) and the pinned magnetic layer, and the spins are switched. The conduction electrons reflected against the interface flow from the nonmagnetic layer again into the free magnetic layer and make the exchange interaction with the electrons of the free magnetic layer. Resultantly, torques are generated between the electrons, and when the torques are sufficiently large, the magnetic moment of the free magnetic layer is switched from parallel to anti-parallel. This switch from parallel to anti-parallel has poor spin transfer efficiency in comparison with the switch from anti-parallel to parallel and requires large current for the switch of the magnetization. As described above, the spin transfer torque switching element induces the magnetization switch of the free magnetic layer by the current control (application direction and application current value) alone to thereby rewrite the memory state.
The spin transfer torque switching element, which has the switching current decreased due to the volume decreasing effect even with the decrease of the element size and the resultant magnetization reversing magnetic field Hc, is much more advantageous to the elements of the current magnetic field write method in the capacity increase and electric power consumption decrease. The spin transfer torque switching element requires no write word lines, which permits the device structure and the fabrication process to be simplified. That is, the MRAM using the spin transfer torque switching elements can use the same device structure as DRAM, which permits the fabrication process to be simplified and the fabrication cost to be accordingly decreased.
However, to realize an MRAM of a large capacity exceeding gigabits, various problems, as of decreasing the write current, etc., are present. Non-Patent Reference 1 describes a cell area of ˜(0.1 μm)2, a RA (element resistance R×cell area A) of ˜tens Ωμm2, resistance change percentage above 30%, a write current (magnetization reversing current) Ic below 0.1 mA, etc. tentatively calculated as the target values for realizing such large-capacity MRAM.
A 0.1 mA write current Ic is equivalent to 1×106 A/cm2 in the critical current density Jc. However, the critical current density Jc so far reported on the research level is 0.5×107˜107 A/cm2 (refer to Non-Patent References 1 and G. D. Fuchs, “Spin-transfer effects in nanoscale magnetic tunnel junctions”, Applied Physics Letters, Vol. 85, No. 7, 2004, pp. 1205-1207 (Non-Patent Reference 2)). Accordingly, to realize a large-capacity MRAM, first, it is necessary to further decrease the write current.
As described above, in the conventional spin transfer torque switching element, the switch from parallel to anti-parallel depending on conduction electrons reflected against the interface between the ferromagnetic layer and the nonmagnetic layer is inefficient in comparison with the switch from anti-parallel to parallel and requires larger current. Accordingly, the resistance-current hysteresis characteristic of the spin transfer torque switching element has poor symmetry.
In the conventional spin transfer torque switching element, the free magnetic layer and the pinned magnetic layer are magnetically coupled due to leaking magnetic field from the pinned magnetic layer, and the resistance-current hysteresis characteristic has shifted.
In the large-capacity MRAM, to make the memory operation stable, high output changes are required. To this end, the spin transfer torque switching element is required to have large resistance change percentage.
The related arts are also disclosed in, e.g., Japanese published unexamined patent application No. 2004-158766 (Patent Reference 2) and Japanese published unexamined patent application No. 2002-359412 (Patent Reference 3).